The present invention relates to a coil tubing injector. More particularly, the present invention relates to a coil tubing injector mounted on a truck with means for evenly reeling the tubing on a storage reel, means for straightening the tubing before injecting it into the well, and means for positioning the injector over the well bore to facilitate injection of the tubing.
Continuous tubing is often used to aid in completion, servicing or production of a well. Often, after the well has been drilled, or even during the drilling process, it is desired to pass a separate tube down the bore hole for passing gasses and fluids down into the hole for a particular purpose. For example, the tubing can be used for the circulation of nitrogen, oil, water, acid, alcohol, chemicals or solvents, for downhole workovers, location of hydrate plugs, placing of cement plugs through packers, and for circulating cement to casing bottoms, among other functions. The placement of the tube in the hole is accomplished by means of a device called an "injector", so-called because the tubing must be forced into the hole until enough of the tubing has been injected that the weight of the tubing inserted into the hole is sufficient to overcome the pressure in the borehole and the resistance to downward movement of the tubing imposed by the straightener.
Normally, the tubing used is a continuous length of tubing without couplings. The use of tubing without couplings decreases the likelihood of rupture of the tubing when injecting gases and fluids into the well hole at extremely high pressures. Also, injection of continuous tubing into the well bore at a steady rate is normally faster than assembling tubing joint by joint for lowering into the hole. Thus, continuous tubing can help save time and drilling costs.
In order to handle and store the continuous tubing, the tubing must be capable of being wound onto a reel or otherwise coiled. If the tubing material is made of PVC pipe or other high-strength plastic, coiling of the tubing for storage poses no significant problems, because the plastic tends to straighten itself when uncoiled for injection into a well bore. However, under certain downhole conditions, more durable materials are required for the tubing. For example, PVC pipe is able to withstand only relatively low pressures. Further, high-strength, low-alloy steel is often used in "sour" environments, i.e., environments in which large amounts of acid or sulfur gases are present. The use of continuous steel pipe which must be stored by coiling poses significant problems because, when uncoiled, the steel pipe tends to retain the curvature imparted to it during storage.
Known tubing injectors consist of a series of moving blocks driven by chains which grip the tubing on opposite sides, pulling it out of storage and and injecting it into the well and straightening it at the same time. However, this type of apparatus for injecting and straightening the tubing often damages the surface of the tubing. Thus, there is a need for a coil tubing injector which both injects and straightens the tubing, but which does not damage the surface of the tubing, thereby extending the life of tubing such as the copper tubing described below, which is relatively expensive to replace. Such a device would be of particular utility for use with special purpose tubing, for example, copper tubing with fiberglass coating such as is used in some segments of the industry for heating thick oil in the well to facilitate production. In these situations, the fiberglass coating is easily damaged by known injecting and straightening devices.
Another limitation of known tubing injectors is the expense of purchasing and maintaining them. By virtue of their size, even second-hand injectors are so expensive to purchase and operate that it is not economical to use them to service moderate or low production wells. In fact, because of this expense, many wells which need to be cemented, an operation which is best carried out by the use of a tubing injector, are not cemented, creating an environmental and safety hazard.
Another problem with known tubing injectors is the interaction between the injecting/straightening unit and the borehole. Because of the many different applications for which tubing may be utilized, the ideal device would be capable of being used on uncased, uncompleted borehole, a producing well which has a well head and "Christmas tree" in place above the borehole, or a well with any other equipment in place. To meet these different operating conditions, without having to alter the well site by removing the Christmas tree or adding well heads, it is desirable that the injecting/straightening unit be capable of operating essentially independently of the well. In other words, to service a producing well, the injecting/straightening unit must be able to rise up over the Christmass tree, some of which are over eight feet high, and operate above it. On the other hand, for an unfinished well, the injecting/straightening unit must operate almost at ground level.
Another consideration in having a tubing injector which operates independently of the equipment on the well is the recent advancement in other areas of oil and gas production in which the tubing injector is used to operate other downhole equipment or as a medium for performing various production tests and remedial operations. When used in this manner, it is desirable that this additional equipment be placed below the injecting and straightening means.
Another problem with known tubing injectors is that the flexibility of their operation is limited by the requirement that the injector be bolted to the well head for support and stability. If the tubing injector is to used on a well before the well head has been attached, or where attachment is inconvenient, a tubing injector which does not need to be attached to the well structure has significant advantages.
U.S. Pat. No. 3,116,781 is directed to a device which injects coil tubing. However, that device is limited in its ability to be adapted to operate over elevated well heads. Further, the utility of that device is limited by the use of the storage reel shown. As tubing is wound on the storage reel as it is retrieved from the well, it will not be distributed across the width of the reel, using its storage capacity to the fullest extent. In an attempt to provide an apparatus which distributes the tubing evenly in storage, injectors have been built with a guide, not unlike the level wind or traverse of a fishing reel, to distribute the tubing evenly onto the reel. However, this design imposes a design limitation on the injector unit which increases the cost and size of the injector unit. To traverse the tubing across the entire width of the reel, it is necessary that the apparatus guiding the tubing be placed at a substantial distance from the reel. Otherwise, when the guiding apparatus moves the tubing to one of the extreme edges of the reel, the tubing will be bent. If the guiding apparatus is positioned on the truck, a greatly increased length of the truck is required, and likewise, an increasingly expensive cost to avoid imparting a bend to the tubing. Further, the increased size creates problems such as the inability of such units to gain access to wells which are, for instance, between structures or other obstacles or in limited working areas.
An example of a design which provides one solution to this design limitation is the tubing injector marketed by Otis Engineering Corporation, which achieves the required distance by mounting the injector/straightener unit on a crane or boom at the rear of the truck or truck trailer on which the unit is mounted. However, the use of a crane creates additional problems such as clearance, increased maintenance and hydraulic system requirements and so forth.
Therefore, it is an object of the present invention to provide a tubing injector which is characterized by its ability to distribute the coil tubing onto a storage means without bending the tubing while still being small enough to be built and operated economically.
It is another object of the present invention to provide a tubing injector comprising a frame with a subframe slidably mounted thereon, a tubing storage means being mounted on the subframe and having coil tubing stored thereon, an injector reel mounted on the frame, means for rotating the injector reel, means mounted around a portion of the circumference of the injector reel for exerting pressure against the coil tubing when the coil tubing is directed between the circumference of the injector reel and said pressure exerting means, means for straightening the tubing, and means for slidably reciprocating the subframe across the frame as the tubing is being returned to the storage means.
It is another object of the present invention to provide a tubing injector unit which does not damage the exterior of the coil tubing.
A further object of the present invention is to provide a tubing injector unit which can operate at different heights above the well.
A further object of the present invention is to provide a tubing injector unit which can be operated without being attached to the well structure.
A further object of the present invention is to provide a tubing injector unit which stores the tubing evenly on a storage reel by traversing the reel as the tubing is withdrawn from the well.
These and other objects of the present invention will be evident to those skilled in the art from the following detailed description of the preferred embodiment.